1. Field of the Invention
The present invention relates to a film of PMMA with a fluorescent substance dispersed therein, to an optical memory material with the film, and to a three-dimensional memory that comprises the optical memory material. The invention also relates to a method for producing the film that comprises PMMA and a fluorescent substance.
2. Description of the Background
The recording capacity of optical memory is greatly increasing these days. This is because of the increasing necessity for it from the development of advanced information society. At present, CD may have 650 Mbytes and single-face one-layer DVD may have4.7 Gbytes, and these recording capacities could not be expected a few years ago. This tendency will go on further, and it may be considered that ultra-mass storage optical memories of a level of T (tera) bytes to P (peta) bytes will be needed in the near future. The advantages of optical memory are the transportability, the long recording life, the resistance to vibration and the mass-reproduction technology. Therefore, optical memory is superior to magnetic recording. In point of the capacity thereof, optical memory is superior to semiconductor memory (smart media, memory sticks). With further popularization of internets, digital videos/cameras, high-definition TVs and others for general home use, optical memory will be much more needed and expected in future.
The recording density of optical memory depends on the capability thereof for light convergence. The size of converging spot is determined by the property as waves of light, and the size is limited to around the wavelength of light owing to the wave diffraction of light. At present, the capability of optical memory is already approaching the physical limit.
A new technical breakthrough is now desired for solution of the problem, for which various methods are now under study. They are (A) a method of using a source of light having a shorter wavelength; (B) a method of using near-field optics for overcoming the limitation on wavelength; and (C) a method of expanding the recording region from two-dimensional plane to three-dimensional space. However, the method (A) of using a source of light having a shorter wavelength is problematic in that semiconductor laser applicable to such light is difficult to develop. In addition, since optical material having a high transmittance within such wavelength range is rare, the method (A) is not practicable. The method (B) of using near-field optics for overcoming the limitation on wavelength is problematic in that, in near-field recording, the distance between the optical head and the recording medium must be near to a few nanometers and it must be kept as such with high accuracy. In addition to the technical difficulty thereof, another drawback of the method is that it could not well utilize the far-running property of light. As opposed to these, the method (C) of recording in a three-dimensional space could be an idea conversion from conventional methods, and it may be a method with hidden potential.
The method of recording information data in a three-dimensional space differs from recording them on one layer alone of a recording medium like in CD, and it comprises positively using the inside of a recording medium for three-dimensionally recording information data in multiple layers of the medium, as in FIG. 17, to thereby increase the space where information data are to be recorded and to increase the recording capacity of the medium. Three-dimensional recording requires a recording medium that enables multi-layer recording, and an optical system that comprises a light source and an objective lens for recording and writing information data in the three-dimensional space of the recording medium and for reproducing the thus-written data. The recording medium and the recording-reproducing optical system must have non-linear responsibility. Only when all these problems with it have been solved, the method will be practicable.
At present, optical engineering technology has made remarkable advances, and it is now possible to solve the above-mentioned problems. Regarding the recording mode, laser light is converged on a recording medium to cause non-linear chemical change of the substance of the medium at the spot position thereof at which the light intensity has increased, whereby the problem with the recording mode may be solved. On the other hand, the problem with the reproduction mode may be solved by employing a laser scanning confocal microscope which has a resolving power also in the depth direction thereof and is characterized in that its in-plane resolving power is two times that of an ordinary incoherent bright-field microscope. For recording materials of non-linear responsibility, heretofore proposed are photopolymers, photo-refractive crystals and photo chromic materials that accept information recording thereon as refractive index distribution. Also proposed are urethane-urea copolymers for them. Various methods of using these recording materials and recording-reproducing optical systems and also three-dimensional optical memories and are now under active study.
For reflection optical memories, known are CD and DVD. For these, employed is a method of forming grooves in accordance with 0/1 information data on the recording surface of the medium, convergent light is applied to the part, and the intensity of the reflected light is read with a detector. In CD-R and DVD-R, a blue-green organic dye is applied on the recording surface of the medium in place of forming the information grooves as in CD and DVD, convergent light is applied to it to burn the dye, and information data are recorded as the burnt pattern in place of the grooves in CD and DVD. On the other hand, in fluorescence recording optical memories, a fluorescent dye-containing recording material is used as the recording medium. For data recording thereon, convergent light is irradiated to the sample to cause chemical change of the fluorescent dye in the irradiated part. Through the chemical change, the fluorescence intensity of the part in excited light irradiation thereto increases or decreases, and based on the fluorescence contrast difference between the irradiated part and the non-irradiated part, 0/1 information data are recorded on the medium.
A fluorescent dot pattern may be recorded in a three-dimensional space by combining the above-mentioned techniques, and this method is for fluorescence recording three-dimensional multi-layer optical memory. The light irradiated from laser is converged inside the recording material that contains a fluorescent sample, through an objective lens, and this is recorded in three-dimensional multiple layers as fluorescent dot data. For reproducing the recorded information data, used is an optical microscope system that enables three-dimensional structure observation. An outline of an episcopic confocal fluorescence microscope is described in point of the mechanism and the optical system thereof, with reference to FIG. 13. The microscope is characterized in that a pinhole is disposed before the detector thereof, and it is known that the microscope has a high three-dimensional resolving power. When excited light is converged on the previously-recorded fluorescent dot data, then it gives fluorescence. The fluorescence passes through an objective lens and is reflected by a beam splitter. The reflected fluorescence passes through the pinhole disposed before the detector, and is then detected by the detector. In that manner, the light from the focal position of the objective lens can pass through the pinhole, but the light having scattered not in the focal position and the fluorescence emitted not therein do not pass through the pinhole but are cut. Accordingly, the optical detector can detect only the fluorescence from the focal position and therefore ensures reading operation with three-dimensional resolution. At present, fluorescence recording optical memories based on the light-emitting and extinguishing property of fluorescent substances have been already reported, and the probability of their application to three-dimensional recording modes is much expected.
A preprint for the 62nd Academic Lecture Meeting of the Applied Physics Society of Japan, September 2001, page 886, and a preprint for the 49th Joint Lecture Meeting of the Applied Physics Society of Japan, March 2002, page 1268 disclose a combination of polymethyl methacrylate (hereinafter referred to as PMMA) and rhodamine B for a material for such optical memories. This utilizes the characteristic of rhodamine B mentioned below.
Specifically, when rhodamine B exists as a form of:
then it exhibits pink, and emits fluorescence through exposure to excited light. On the other hand, when it exists as a form of:
then it is colorless and does not emit fluorescence. In this connection, J. Muto, F. Higuchi, Phys. Lett., 96A, No. 2, 101 (1983) says that the color presentation and the light emission of the compound are both owing to ring cleavage/ring closing of the lactone ring of the compound.
Some study cases of utilizing the above-mentioned mechanism for recording media of three-dimensional optical memories have been reported. In the preprint for the 62nd Academic Lecture Meeting of the Applied Physics Society of Japan, September 2001, page 886, announced is a study report saying that a monomer methyl methacrylate (hereinafter referred to as MMA), a polymerization initiator MMA polymer, tetrachloroauric acid and rhodamine B are formulated, and the resulting solution is solidified on a cover glass to fabricate a recording medium. In this method, when 3-valent gold ions exist near rhodamine B molecules, they absorb the energy in light excitation and retard fluorescence emission. When the recording medium is irradiated with UV light, then the 3-valent gold ions in the irradiated area are reduced to give gold particles. The gold particles could not absorb the energy of excited light and therefore the irradiated area may emit fluorescence. The ON-OFF switching of UV light makes it possible to record a fluorescence pattern on the recording medium.
The preprint for the 49th Joint Lecture Meeting of the Applied Physics Society of Japan, March2002, page 1268 discloses PMMA doped with rhodamine B in the absence of chloroauric acid. Specifically, a polymer formed through polymerization of a solution of a monomer MMA, rhodamine B and a polymerization initiator is known. When the polymer is irradiated with UV light and further with excited green light, then the irradiated part alone thereof emits fluorescence. The ON-OFF switching of UV light makes it possible to record a fluorescence pattern on the recording medium.
This is described concretely with reference to FIG. 18. a) Rhodamine B dissolved in a solvent is generally red, and it has an absorption peak at a wavelength of 542.8 nm and emits fluorescence. b) When rhodamine B is doped into PMMA, then it loses its color and fades to be colorless transparent. In this condition, even when it is irradiated with excited light, it does not emit fluorescence. c) When the rhodamine B-doped PMMA is irradiated with UV light, then the rhodamine B is re-activated in the irradiated area and again emits pink. d) When the UV-irradiated part is further irradiated with excited light, then it emits fluorescence. Rhodamine B has an absorption peak at a wavelength of 542.8 nm and has a fluorescence peak at a wavelength of 565 nm.
Through our studies, however, we, the present inventors have found that it is extremely difficult to use the above-mentioned material that contains PMMA and rhodamine B for a memory material. Specifically, the above-mentioned composition that contains PMMA and rhodamine B could not be formed into films in conventional techniques. For example, even when the composition is formed into film in a mode of spin coating, all its components immediately evaporate away. This is because the viscosity of the starting MMA for PMMA is originally extremely low and it is a highly-volatile liquid. Accordingly, even when the method of polymerizing MMA to give PMMA is utilized, only a composition containing PMMA which is solid in some degree and rhodamine B could be obtained.
Through our further studies, we have found that, when MMA is polymerized according to conventional methods, then the degree of polymerization of the polymer PMMA formed is unclear and is extremely uneven, and it is difficult to readily control the degree of polymerization of the polymer. Therefore, we have found that, even when MMA is polymerized, it is still impossible to produce a polymer film having a uniform surface and having a uniform thickness.
We actually investigated a method of forming a thin mass relatively near to a film, according to the above-mentioned method. Concretely, a space having a width of a few millimeters or so was formed of glass slides, and the above-mentioned polymerization was tried in the space. In this case, however, bubbles were formed in the resin and the resin became cloudy. In addition, rhodamine B was denatured owing to the heat generated by MMA polymerization, and the resulting polymer could not undergo the mechanism of fluorescence emission/extinction in many cases. Accordingly, it was impossible to obtain films from the composition that contains PMMA and rhodamine B according to conventional methods. In particular, the composition that may fail to undergo the mechanism of fluorescence emission/extinction is a serious defect in optical memory materials.